I know something very similar happened in the Masters in 2005.
This is actually really interesting.

If I'm not wrong:
1) If Marion goes 2-0, she automatically tops the group. The other players can come up with a 2-1 at best so Marion goes on top based on percentages.
2) If Marion loses to Justine and beats Jelena, and Anna loses to Jelena, she goes 1-1 while Jelena and Anna go 1-2. So Marion will come in second in the group based on percentages again.

Can someone correct me if I'm wrong?

In case 1, suppose Anna beats Jelena. Both Justine and Anna will end up with 2-1. Sets won/lost become crucial. Does Anna's win over Serena count as a 1-0 or a 2-0 for her sets record?

A single flow'r he sent me, since we met./All tenderly his messenger he chose;
Deep-hearted, pure, with scented dew still wet - One perfect rose.I knew the language of the floweret;/'My fragile leaves,' it said, 'his heart enclose.'
Love long has taken for his amulet/One perfect rose.Why is it no one ever sent me yet/One perfect limousine, do you suppose?
Ah no, it's always just my luck to get/One perfect rose.

I know something very similar happened in the Masters in 2005.
This is actually really interesting.

If I'm not wrong:
1) If Marion goes 2-0, she automatically tops the group. The other players can come up with a 2-1 at best so Marion goes on top based on percentages.
2) If Marion loses to Justine and beats Jelena, and Anna loses to Jelena, she goes 1-1 while Jelena and Anna go 1-2. So Marion will come in second in the group based on percentages again.

Can someone correct me if I'm wrong?

In case 1, suppose Anna beats Jelena. Both Justine and Anna will end up with 2-1. Sets won/lost become crucial. Does Anna's win over Serena count as a 1-0 or a 2-0 for her sets record?

I think that in case of the same number of wins, the player who competed in more matches will qualify.